A laboratory fume hood is a ventilated enclosure where harmful materials can be handled safely. The hood captures contaminants and prevents them from escaping into the laboratory by using an exhaust blower to draw air and contaminants in and around the hood's work area away from the operator so that inhalation of and contact with the contaminants are minimized. Access to the interior of the hood is through an opening which is closed with one or more sashes which may slide vertically, horizontally, or in both directions to vary the opening into the hood.
A conventional fume hood consists of an enclosure which forms five sides of the hood and a hood sash or sashes which slide horizontally and/or vertically to provide a variable-sized opening on the sixth side. In this type of hood, the amount of air exhausted by the hood blower is essentially fixed and the velocity of air flow through the hood opening, or face velocity, increases as the area of the sash opening decreases. As a result, the sash must be left open an appreciable amount even when the hood is not being used by an operator to allow air to enter the hood opening at a reasonable velocity. However, as is discussed in U.S. Pat. Nos. 4,528,898 and 4,706,555, the amount of energy required to deliver "make up air" may be reduced by monitoring the sash position, and thus the opening in the fume hood and by adjusting the blower and thus the exhaust volume of the hood linearly in proportion to the change in opening size in order to achieve a substantially constant face velocity. In these patents, the fume hood opening was covered by a single sash which opened in the vertical direction.
U.S. Pat. No. 4,893,551 discusses additional styles of fume hoods wherein two or more sashes are mounted to slide horizontally on at least two tracks which are located on the top and bottom of the sash opening and also fume hoods which have sashes mounted on tracks for horizontal movement, which tracks are, in turn, mounted on a sash frame which may be moved vertically. This patent also discusses techniques which may be utilized with such sashes to determine the sash opening. As is noted in this patent, with two or more sashes, absolute position of the sashes is not sufficient information by itself to indicate the open area of the hood. Instead, it is the relative position of the two or more sashes of the hood which determine the total open sash area. The problem becomes even more complex where four sashes are mounted on two tracks, which is a very common configuration, or where the hood is being moved both horizontally and vertically.
In the U.S. Pat. No. 4,893,551 patent, the sash opening detection function is performed, in general, by having a source of radiation, and a detector for such radiation, and by mounting the source and detector relative to each other and to the sashes such that the amount of radiation detected is proportional to the uncovered portion of the opening. For preferred embodiments in the patent, various discrete magnetic or optical emitters and sensors mounted adjacent to or on the sashes are utilized to determine the fume hood opening.
However, the detectors, and in some cases the sources, for these preferred embodiments utilize active devices which may need to be installed inside or near the opening of the fume hood. This results in a need for careful sealing of these devices with the attendant cost and complexity. These active devices, and even some of the nonactive devices disclosed in the patent, also require an enclosure having a reasonable thickness, particularly when sealing is required. This can cause problems in locating such devices on the sashes of some hoods. In particular, such devices may not fit within the clearance between the sashes or between the sashes and the frame of the hood.
Further, the preferred embodiments in the patent utilize a number of discrete components, and, therefore, provide discrete outputs rather than a continuous output. The degree of precision with such apparatus depends on the number of sensors utilized and is generally hot better than about one-half inch. Even to achieve this degree of precision, a large number of discrete sources and detectors are required which results in the apparatus being relatively complex and expensive. The increased number of apparatus also results in an enhanced likelihood of component failure.
A need, therefore, exists for improved embodiments for such fume hood sash sensing apparatus which do not require the use of active devices and which may be fabricated to be very thin. It would also be desirable if at least some such embodiments could provide continuous rather than discrete outputs. Finally, it would be desirable if discrete components could be substantially eliminated so as to enhance the reliability of the apparatus.